Mitotic phosphatase activity is required for MCC maintenance during the spindle checkpoint

The spindle checkpoint prevents activation of the anaphase-promoting complex (APC/C) until all chromosomes are correctly attached to the mitotic spindle. Early in mitosis, the mitotic checkpoint complex (MCC) inactivates the APC/C by binding the APC/C activating protein CDC20 until the chromosomes are properly aligned and attached to the mitotic spindle, at which point MCC disassembly releases CDC20 to activate the APC/C. Once the APC/C is activated, it targets cyclin B and securin for degradation, and the cell progresses into anaphase. While phosphorylation is known to drive many of the events during the checkpoint, the precise molecular mechanisms regulating spindle checkpoint maintenance and inactivation are still poorly understood. We sought to determine the role of mitotic phosphatases during the spindle checkpoint. To address this question, we treated spindle checkpoint-arrested cells with various phosphatase inhibitors and examined the effect on the MCC and APC/C activation. Using this approach we found that 2 phosphatase inhibitors, calyculin A and okadaic acid (1 μM), caused MCC dissociation and APC/C activation leading to cyclin A and B degradation in spindle checkpoint-arrested cells. Although the cells were able to degrade cyclin B, they did not exit mitosis as evidenced by high levels of Cdk1 substrate phosphorylation and chromosome condensation. Our results provide the first evidence that phosphatases are essential for maintenance of the MCC during operation of the spindle checkpoint.

An essential step of kinetochore formation controlled by the SNARE protein Snap29

The kinetochore is an essential structure that mediates accurate chromosome segregation in mitosis and meiosis. While many of the kinetochore components have been identified, the mechanisms of kinetochore assembly remain elusive. Here, we identify a novel role for Snap29, an unconventional SNARE, in promoting kinetochore assembly during mitosis in Drosophila and human cells. Snap29 localizes to the outer kinetochore and prevents chromosome mis‐segregation and the formation of cells with fragmented nuclei. Snap29 promotes accurate chromosome segregation by mediating the recruitment of Knl1 at the kinetochore and ensuring stable microtubule attachments. Correct Knl1 localization to kinetochore requires human or Drosophila Snap29, and is prevented by a Snap29 point mutant that blocks Snap29 release from SNARE fusion complexes. Such mutant causes ectopic Knl1 recruitment to trafficking compartments. We propose that part of the outer kinetochore is functionally similar to membrane fusion interfaces.

Bub3-Bub1 Binding to Spc7/KNL1 Toggles the Spindle Checkpoint Switch by Licensing the Interaction of Bub1 with Mad1-Mad2

The spindle assembly checkpoint (SAC) ensures that sister chromatids do not separate until all chromosomes are attached to spindle microtubules and bi-oriented. Spindle checkpoint proteins, including Mad1, Mad2, Mad3 (BubR1), Bub1, Bub3, and Mph1 (Mps1), are recruited to unattached and/or tensionless kinetochores. SAC activation catalyzes the conversion of soluble Mad2 (O-Mad2) into a form (C-Mad2) that binds Cdc20, BubR1, and Bub3 to form the mitotic checkpoint complex (MCC), a potent inhibitor of the anaphase-promoting complex (APC/C). SAC silencing de-represses Cdc20-APC/C activity allowing poly-ubiquitination of Securin and Cyclin B, leading to the dissolution of sister chromatids and anaphase onset [1]. Understanding how microtubule interaction at kinetochores influences the timing of anaphase requires an understanding of how spindle checkpoint protein interaction with the kinetochore influences spindle checkpoint signaling. We, and others, recently showed that Mph1 (Mps1) phosphorylates multiple conserved MELT motifs in the Spc7 (Spc105/KNL1) protein to recruit Bub1, Bub3, and Mad3 (BubR1) to kinetochores [2-4]. In budding yeast, Mps1 phosphorylation of a central non-catalytic region of Bub1 promotes its association with the Mad1-Mad2 complex, although this association has not yet been detected in other organisms [5]. Here we report that multisite binding of Bub3 to the Spc7 MELT array toggles the spindle checkpoint switch by permitting Mph1 (Mps1)-dependent interaction of Bub1 with Mad1-Mad2.

The RZZ complex requires the N-terminus of KNL1 to mediate optimal Mad1 kinetochore localization in human cells

The spindle assembly checkpoint is a surveillance mechanism that blocks anaphase onset until all chromosomes are properly attached to microtubules of the mitotic spindle. Checkpoint activity requires kinetochore localization of Mad1/Mad2 to inhibit activation of the anaphase promoting complex/cyclosome in the presence of unattached kinetochores. In budding yeast and Caenorhabditis elegans, Bub1, recruited to kinetochores through KNL1, recruits Mad1/Mad2 by direct linkage with Mad1. However, in human cells it is not yet established which kinetochore protein(s) function as the Mad1/Mad2 receptor. Both Bub1 and the RZZ complex have been implicated in Mad1/Mad2 kinetochore recruitment; however, their specific roles remain unclear. Here, we investigate the contributions of Bub1, RZZ and KNL1 to Mad1/Mad2 kinetochore recruitment. We find that the RZZ complex localizes to the N-terminus of KNL1, downstream of Bub1, to mediate robust Mad1/Mad2 kinetochore localization. Our data also point to the existence of a KNL1-, Bub1-independent mechanism for RZZ and Mad1/Mad2 kinetochore recruitment. Based on our results, we propose that in humans, the primary mediator for Mad1/Mad2 kinetochore localization is the RZZ complex.

The Molecular Biology of Spindle Assembly Checkpoint Signaling Dynamics

The spindle assembly checkpoint is a safeguard mechanism that coordinates cell-cycle progression during mitosis with the state of chromosome attachment to the mitotic spindle. The checkpoint prevents mitotic cells from exiting mitosis in the presence of unattached or improperly attached chromosomes, thus avoiding whole-chromosome gains or losses and their detrimental effects on cell physiology. Here, I review a considerable body of recent progress in the elucidation of the molecular mechanisms underlying checkpoint signaling, and identify a number of unresolved questions.

Two functionally distinct kinetochore pools of BubR1 ensure accurate chromosome segregation

The BubR1/Bub3 complex is an important regulator of chromosome segregation as it facilitates proper kinetochore–microtubule interactions and is also an essential component of the spindle assembly checkpoint (SAC). Whether BubR1/Bub3 localization to kinetochores in human cells stimulates SAC signalling or only contributes to kinetochore–microtubule interactions is debated. Here we show that two distinct pools of BubR1/Bub3 exist at kinetochores and we uncouple these with defined BubR1/Bub3 mutants to address their function. The major kinetochore pool of BubR1/Bub3 is dependent on direct Bub1/Bub3 binding and is required for chromosome alignment but not for the SAC. A distinct pool of BubR1/Bub3 localizes by directly binding to phosphorylated MELT repeats on the outer kinetochore protein KNL1. When we prevent the direct binding of BubR1/Bub3 to KNL1 the checkpoint is weakened because BubR1/Bub3 is not incorporated into checkpoint complexes efficiently. In conclusion, kinetochore localization supports both known functions of BubR1/Bub3.

The BubR1/Bub3 complex regulates chromosome segregation to enable proper kinetochore-microtubule interactions and is also required for the spindle assembly checkpoint. Here the authors show that two distinct pools of BubR1/Bub3 exist at kinetochores to support both known functions of BubR1/Bub3.

Dual mechanisms regulate the recruitment of spindle assembly checkpoint proteins to the budding yeast kinetochore

Quantitative knowledge of the recruitment of spindle assembly checkpoint (SAC) proteins by the kinetochore is essential to understanding the mechanisms that regulate protein recruitment and hence the strength of the SAC. Here this recruitment is quantified, and novel mechanisms are identified that strongly modulate SAC protein recruitment by the kinetochore.

Recruitment of spindle assembly checkpoint (SAC) proteins by an unattached kinetochore leads to SAC activation. This recruitment is licensed by the Mps1 kinase, which phosphorylates the kinetochore protein Spc105 at one or more of its six MELT repeats. Spc105 then recruits the Bub3-Bub1 and Mad1-Mad2 complexes, which produce the inhibitory signal that arrests cell division. The strength of this signal depends, in part, on the number of Bub3-Bub1 and Mad1-Mad2 molecules that Spc105 recruits. Therefore regulation of this recruitment will influence SAC signaling. To understand this regulation, we established the physiological binding curves that describe the binding of Bub3-Bub1 and Mad1-Mad2 to the budding yeast kinetochore. We find that the binding of both follows the mass action law. Mps1 likely phosphorylates all six MELT repeats of Spc105. However, two mechanisms prevent Spc105 from recruiting six Bub3-Bub1 molecules: low Bub1 abundance and hindrance in the binding of more than one Bub3-Bub1 molecule to the same Spc105. Surprisingly, the kinetochore recruits two Mad1-Mad2 heterotetramers for every Bub3-Bub1 molecule. Finally, at least three MELT repeats per Spc105 are needed for accurate chromosome segregation. These data reveal that kinetochore-intrinsic and -extrinsic mechanisms influence the physiological operation of SAC signaling, potentially to maximize chromosome segregation accuracy.

The human SKA complex drives the metaphase-anaphase cell cycle transition by recruiting protein phosphatase 1 to kinetochores

The spindle- and kinetochore-associated (Ska) complex is essential for normal anaphase onset in mitosis. The C-terminal domain (CTD) of Ska1 binds microtubules and was proposed to facilitate kinetochore movement on depolymerizing spindle microtubules. Here, we show that Ska complex recruits protein phosphatase 1 (PP1) to kinetochores. This recruitment requires the Ska1 CTD, which binds PP1 in vitro and in human HeLa cells. Ska1 lacking its CTD fused to a PP1-binding peptide or fused directly to PP1 rescues mitotic defects caused by Ska1 depletion. Ska1 fusion to catalytically dead PP1 mutant does not rescue and shows dominant negative effects. Thus, the Ska complex, specifically the Ska1 CTD, recruits PP1 to kinetochores to oppose spindle checkpoint signaling kinases and promote anaphase onset. Microtubule binding by Ska, rather than acting in force production for chromosome movement, may instead serve to promote PP1 recruitment to kinetochores fully attached to spindle microtubules at metaphase.

Mps1Mph1 Kinase Phosphorylates Mad3 to Inhibit Cdc20Slp1-APC/C and Maintain Spindle Checkpoint Arrests

The spindle checkpoint is a mitotic surveillance system which ensures equal segregation of sister chromatids. It delays anaphase onset by inhibiting the action of the E3 ubiquitin ligase known as the anaphase promoting complex or cyclosome (APC/C). Mad3/BubR1 is a key component of the mitotic checkpoint complex (MCC) which binds and inhibits the APC/C early in mitosis. Mps1^Mph1 kinase is critical for checkpoint signalling and MCC-APC/C inhibition, yet few substrates have been identified. Here we identify Mad3 as a substrate of fission yeast Mps1^Mph1 kinase. We map and mutate phosphorylation sites in Mad3, producing mutants that are targeted to kinetochores and assembled into MCC, yet display reduced APC/C binding and are unable to maintain checkpoint arrests. We show biochemically that Mad3 phospho-mimics are potent APC/C inhibitors in vitro, demonstrating that Mad3p modification can directly influence Cdc20^Slp1-APC/C activity. This genetic dissection of APC/C inhibition demonstrates that Mps1^Mph1 kinase-dependent modifications of Mad3 and Mad2 act in a concerted manner to maintain spindle checkpoint arrests.

When cells divide they need to ensure that a complete copy of their genetic material is transmitted to both daughter cells. Cells have evolved many controls to ensure that every division is carried out with very high fidelity. The spindle checkpoint is one such control, which acts as a surveillance system during mitosis. Defects in this checkpoint control lead to unequal segregation of DNA/chromosomes, termed aneuploidy, which is responsible for human birth defects and is very common in tumour cells.

The molecular components of the spindle checkpoint, identified initially through yeast genetics, include several protein kinases. Surprisingly few of their substrates have been identified. Here we identify the checkpoint protein Mad3 as an important substrate of the Mps1^Mph1 kinase. We show that Mps1^Mph1-dependent modification of Mad3 and Mad2 acts to delay cell division in situations where the genetic material would not be equally inherited by daughter cells. This delay enables the cell to correct any problems within the division machinery and thus avoid aneuploidy.

How the SAC gets the axe: Integrating kinetochore microtubule attachments with spindle assembly checkpoint signaling

Mitosis entails the bona fide segregation of duplicated chromosomes. This process is accomplished by the attachment of kinetochores on chromosomes to microtubules (MTs) of the mitotic spindle. Once the appropriate attachment is achieved, the spindle assembly checkpoint (SAC) that delays the premature onset of anaphase needs to be silenced for the cell to proceed to anaphase and cytokinesis. Therefore, while it is imperative to preserve the SAC when kinetochores are unattached, it is of paramount importance that SAC components are removed post kinetochore microtubule (kMT) attachment. Precise knowledge of how kMT attachments trigger the removal of SAC components from kinetochores or how the checkpoint proteins feedback in to the attachment machinery remains elusive. This review aims to describe the recent advances that provide an insight into the interplay of molecular events that coordinate and regulate the SAC activity in response to kMT attachment during cell division.

Sharpening the anaphase switch

The segregation of sister chromatids during mitosis is one of the most easily visualized, yet most remarkable, events during the life cycle of a cell. The accuracy of this process is essential to maintain ploidy during cell duplication. Over the past 20 years, substantial progress has been made in identifying components of both the kinetochore and the mitotic spindle that generate the force to move mitotic chromosomes. Additionally, we now have a reasonable, albeit incomplete, understanding of the molecular and biochemical events that are involved in establishing and dissolving sister-chromatid cohesion. However, it is less well-understood how this dissolution of cohesion occurs synchronously on all chromosomes at the onset of anaphase. At the centre of the action is the anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase that, in association with its activator cell-division cycle protein 20 homologue (Cdc20), is responsible for the destruction of securin. This leads to the activation of separase, a specialized protease that cleaves the kleisin-subunit of the cohesin complex, to relieve cohesion between sister chromatids. APC/C-Cdc20 is also responsible for the destruction of cyclin B and therefore inactivation of the cyclin B-cyclin-dependent kinase 1 (Cdk1). This latter event induces a change in the microtubule dynamics that results in the movement of sister chromatids to spindle poles (anaphase A), spindle elongation (anaphase B) and the onset of cytokinesis. In the present paper, we review the emerging evidence that multiple, spatially and temporally regulated feedback loops ensure anaphase onset is rapid, co-ordinated and irreversible.

Bub1 autophosphorylation feeds back to regulate kinetochore docking and promote localized substrate phosphorylation

During mitosis, Bub1 kinase phosphorylates histone H2A-T120 to promote centromere sister chromatid cohesion through recruitment of shugoshin (Sgo) proteins. The regulation and dynamics of H2A-T120 phosphorylation are poorly understood. Using quantitative phosphoproteomics we show that Bub1 is autophosphorylated at numerous sites. We confirm mitosis-specific autophosphorylation of a several residues and show that Bub1 activation is primed in interphase but fully achieved only in mitosis. Mutation of a single autophosphorylation site T589 alters kinetochore turnover of Bub1 and results in uniform H2A-T120 phosphorylation and Sgo recruitment along chromosome arms. Consequently, improper sister chromatid resolution and chromosome segregation errors are observed. Kinetochore tethering of Bub1-T589A refocuses H2A-T120 phosphorylation and Sgo1 to centromeres. Recruitment of the Bub1-Bub3-BubR1 axis to kinetochores has recently been extensively studied. Our data provide novel insight into the regulation and kinetochore residency of Bub1 and indicate that its localization is dynamic and tightly controlled through feedback autophosphorylation.

Natural Loss of Mps1 Kinase in Nematodes Uncovers a Role for Polo-like Kinase 1 in Spindle Checkpoint Initiation

The spindle checkpoint safeguards against chromosome loss during cell division by preventing anaphase onset until all chromosomes are attached to spindle microtubules. Checkpoint signal is generated at kinetochores, the primary attachment site on chromosomes for spindle microtubules. Mps1 kinase initiates checkpoint signaling by phosphorylating the kinetochore-localized scaffold protein Knl1 to create phospho-docking sites for Bub1/Bub3. Mps1 is widely conserved but is surprisingly absent in many nematode species. Here, we show that PLK-1, which targets a substrate motif similar to that of Mps1, functionally substitutes for Mps1 in C. elegans by phosphorylating KNL-1 to direct BUB-1/BUB-3 kinetochore recruitment. This finding led us to re-examine checkpoint initiation in human cells, where we found that Plk1 co-inhibition significantly reduced Knl1 phosphorylation and Bub1 kinetochore recruitment relative to Mps1 inhibition alone. Thus, the finding that PLK-1 functionally substitutes for Mps1 in checkpoint initiation in C. elegans uncovered a role for Plk1 in species that have Mps1.

Distinct domains in Bub1 localize RZZ and BubR1 to kinetochores to regulate the checkpoint

The spindle assembly checkpoint (SAC) ensures proper chromosome segregation by delaying anaphase onset in response to unattached kinetochores. Checkpoint signalling requires the kinetochore localization of the Mad1–Mad2 complex that in more complex eukaryotes depends on the Rod–Zwilch–ZW10 (RZZ) complex. The kinetochore protein Zwint has been proposed to be the kinetochore receptor for RZZ, but here we show that Bub1 and not Zwint is required for RZZ recruitment. We find that the middle region of Bub1 encompassing a domain essential for SAC signalling contributes to RZZ localization. In addition, we show that a distinct region in Bub1 mediates kinetochore localization of BubR1 through direct binding, but surprisingly removal of this region increases checkpoint strength. Our work thus uncovers how Bub1 coordinates checkpoint signalling by distinct domains for RZZ and BubR1 recruitment and suggests that Bub1 localizes antagonistic checkpoint activities.

The spindle assembly checkpoint (SAC) depends on the recruitment of specific protein complexes to the kinetochore. Here Zhang et al. show that Bub1 recruits the RZZ complex and BubR1 to the kinetochore, and loss of the BubR1 binding sequence enhances checkpoint activity suggesting both SAC activating and silencing roles.

Kinetochore components are required for central spindle assembly

A critical structure poised to coordinate chromosome segregation with division plane specification is the central spindle that forms between separating chromosomes after anaphase onset. The central spindle acts as a signalling centre that concentrates proteins essential for division plane specification and contractile ring constriction. However, the molecular mechanisms that control the initial stages of central spindle assembly remain elusive. Using Caenorhabditis elegans zygotes, we found that the microtubule-bundling protein SPD-1(PRC1) and the motor ZEN-4(MKLP-1) are required for proper central spindle structure during its elongation. In contrast, we found that the kinetochore controls the initiation of central spindle assembly. Specifically, central spindle microtubule assembly is dependent on kinetochore recruitment of the scaffold protein KNL-1, as well as downstream partners BUB-1, HCP-1/2(CENP-F) and CLS-2(CLASP); and is negatively regulated by kinetochore-associated protein phosphatase 1 activity. This in turn promotes central spindle localization of CLS-2(CLASP) and initial central spindle microtubule assembly through its microtubule polymerase activity. Together, our results reveal an unexpected role for a conserved kinetochore protein network in coupling two critical events of cell division: chromosome segregation and cytokinesis.

A motif from Lys216 to Lys222 in human BUB3 protein is a nuclear localization signal and critical for BUB3 function in mitotic checkpoint

Human BUB3 is a key mitotic checkpoint factor that recognizes centromeric components and recruits other mitotic checkpoint molecules to the unattached kinetochore. The key amino acid residues responsible for its localization are not yet defined. In this study, we identified a motif from Lys(216) to Lys(222) in BUB3 as its nuclear localization signal. A BUB3 mutant with deletion of this motif (Del216-222) was found to localize to both the cytoplasm and the nucleus, distinct from the exclusively nuclear distribution of wild-type BUB3. Further analysis revealed that residues Glu(213), Lys(216), Lys(217), Lys(218), Tyr(219), and Phe(221), but not Lys(222), contribute to nuclear localization. Interestingly, the nuclear localization signal was also critical for the kinetochore localization of BUB3. The deletion mutant Del216-222 and a subtle mutant with four residue changes in this region (E213Q/K216E/K217E/K218E (QE)) did not localize to the kinetochore efficiently or mediate mitotic checkpoint arrest. Protein interaction data suggested that the QE mutant was able to interact with BUB1, MAD2, and BubR1 but that its association with the centromeric components CENP-A and KNL1 was impaired. A motif from Leu(61) to Leu(65) in CENP-A was found to be involved in the association of BUB3 and CENP-A in cells; however, further assays suggested that CENP-A does not physically interact with BUB3 and does not affect BUB3 localization. Our findings help to dissect the mechanisms of BUB3 in mitotic checkpoint signaling.

Sequential multisite phospho-regulation of KNL1-BUB3 interfaces at mitotic kinetochores

Regulated recruitment of the kinase-adaptor complex BUB1/BUB3 to kinetochores is crucial for correcting faulty chromosome-spindle attachments and for spindle assembly checkpoint (SAC) signaling. BUB1/BUB3 localizes to kinetochores by binding phosphorylated MELT motifs (MELpT) in the kinetochore scaffold KNL1. Human KNL1 has 19 repeats that contain a MELT-like sequence. The repeats are, however, larger than MELT, and repeat sequences can vary significantly. Using systematic screening, we show that only a limited number of repeats is "active." Repeat activity correlates with the presence of a vertebrate-specific SHT motif C-terminal to the MELT sequence. SHT motifs are phosphorylated by MPS1 in a manner that requires prior phosphorylation of MELT. Phospho-SHT (SHpT) synergizes with MELpT in BUB3/BUB1 binding in vitro and in cells, and human BUB3 mutated in a predicted SHpT-binding surface cannot localize to kinetochores. Our data show sequential multisite regulation of the KNL1-BUB1/BUB3 interaction and provide mechanistic insight into evolution of the KNL1-BUB3 interface.

Regulation of mitotic progression by the spindle assembly checkpoint

Equal segregation of sister chromatids during mitosis requires that pairs of kinetochores establish proper attachment to microtubules emanating from opposite poles of the mitotic spindle. The spindle assembly checkpoint (SAC) protects against errors in segregation by delaying sister separation in response to improper kinetochore–microtubule interactions, and certain checkpoint proteins help to establish proper attachments. Anaphase entry is inhibited by the checkpoint through assembly of the mitotic checkpoint complex (MCC) composed of the 2 checkpoint proteins, Mad2 and BubR1, bound to Cdc20. The outer kinetochore acts as a catalyst for MCC production through the recruitment and proper positioning of checkpoint proteins and recently there has been remarkable progress in understanding how this is achieved. Here, we highlight recent advances in our understanding of kinetochore–checkpoint protein interactions and inhibition of the anaphase promoting complex by the MCC.

Dissecting the roles of human BUB1 in the spindle assembly checkpoint

Mitotic chromosome segregation is initiated by the anaphase promoting complex/cyclosome (APC/C) and its co-activator CDC20. APC/CCDC20 is inhibited by the spindle assembly checkpoint (SAC) when chromosomes have not attached to spindle microtubules. Unattached kinetochores catalyze the formation of a diffusible APC/CCDC20 inhibitor that is composed of BUBR1, BUB3, MAD2 and a second molecule of CDC20. Kinetochore recruitment of these proteins as well as SAC activation rely on the mitotic kinase BUB1, but the molecular mechanism by which BUB1 accomplishes this in human cells is unknown. We show that BUBR1 and BUB3 kinetochore recruitment by BUB1 is dispensable for SAC activation. Unlike its yeast and nematode orthologs, human BUB1 does not associate stably with the MAD2 activator MAD1 and, although required for accelerating loading of MAD1 onto kinetochores, is dispensable for its steady-state levels there. Instead, we identify a 50 amino acid segment harboring the recently reported ABBA motif close to a KEN box as critical for BUB1's role in SAC signaling. The presence of this segment correlates with SAC activity and efficient binding of CDC20 but not MAD1 to kinetochores.

The outer kinetochore protein KNL-1 contains a defined oligomerization domain in nematodes

The kinetochore is a large, macromolecular assembly that is essential for connecting chromosomes to microtubules during mitosis. Despite the recent identification of multiple kinetochore components, the nature and organization of the higher-order kinetochore structure remain unknown. The outer kinetochore KNL-1/Mis12 complex/Ndc80 complex (KMN) network plays a key role in generating and sensing microtubule attachments. Here we demonstrate that Caenorhabditis elegans KNL-1 exists as an oligomer, and we identify a specific domain in KNL-1 responsible for this activity. An N-terminal KNL-1 domain from both C. elegans and the related nematode Caenorhabditis remanei oligomerizes into a decameric assembly that appears roughly circular when visualized by electron microscopy. On the basis of sequence and mutational analysis, we identify a small hydrophobic region as responsible for this oligomerization activity. However, mutants that precisely disrupt KNL-1 oligomerization did not alter KNL-1 localization or result in the loss of embryonic viability based on gene replacements in C. elegans. In C. elegans, KNL-1 oligomerization may coordinate with other kinetochore activities to ensure the proper organization, function, and sensory capabilities of the kinetochore-microtubule attachment.

Negative feedback at kinetochores underlies a responsive spindle checkpoint signal

Kinetochores are specialised multi-protein complexes that play a crucial role in maintaining genome stability 1. They bridge attachments between chromosomes and microtubules during mitosis and they activate the spindle assembly checkpoint (SAC) to arrest division until all chromosomes are attached 2. Kinetochores are able to efficiently integrate these two processes because they can rapidly respond to changes in microtubule occupancy by switching localised SAC signalling ON or OFF 2–4. We show that this responsiveness arises because the SAC primes kinetochore phosphatases to induce negative feedback and silence its own signal. Active SAC signalling recruits PP2A-B56 to kinetochores where it antagonises Aurora B to promote PP1 recruitment. PP1 in turn silences the SAC and delocalises PP2A-B56. Preventing or bypassing key regulatory steps demonstrates that this spatiotemporal control of phosphatase feedback underlies rapid signal switching at the kinetochore by; 1) allowing the SAC to quickly transition to the ON state in the absence of antagonising phosphatase activity, and 2) ensuring phosphatases are then primed to rapidly switch the SAC signal OFF when kinetochore kinase activities are diminished by force-producing microtubule attachments.

Signalling dynamics in the spindle checkpoint response

The spindle checkpoint ensures proper chromosome segregation during cell division. Unravelling checkpoint signalling has been a long-standing challenge owing to the complexity of the structures and forces that regulate chromosome segregation. New reports have now substantially advanced our understanding of checkpoint signalling mechanisms at the kinetochore, the structure that connects microtubules and chromatin. In contrast to the traditional view of a binary checkpoint response - either completely on or off - new findings indicate that the checkpoint response strength is variable. This revised perspective provides insight into how checkpoint bypass can lead to aneuploidy and informs strategies to exploit these errors for cancer treatments.

Substrate-specific activation of the mitotic kinase Bub1 through intramolecular autophosphorylation and kinetochore targeting

During mitosis of human cells, the kinase Bub1 orchestrates chromosome segregation through phosphorylating histone H2A and the anaphase-promoting complex/cyclosome activator Cdc20. Bub1-mediated H2A-T120 phosphorylation (H2A-pT120) at kinetochores promotes centromeric sister-chromatid cohesion, whereas Cdc20 phosphorylation by Bub1 contributes to spindle checkpoint signaling. Here, we show that phosphorylation at the P+1 substrate-binding loop of human Bub1 enhances its activity toward H2A but has no effect on its activity toward Cdc20. We determine the crystal structure of phosphorylated Bub1. A comparison between structures of phosphorylated and unphosphorylated Bub1 reveals phosphorylation-triggered reorganization of the P+1 loop. This activating phosphorylation of Bub1 is constitutive during the cell cycle. Enrichment of H2A-pT120 at mitotic kinetochores requires kinetochore targeting of Bub1. The P+1 loop phosphorylation of Bub1 appears to occur through intramolecular autophosphorylation. Our study provides structural and functional insights into substrate-specific regulation of a key mitotic kinase and expands the repertoire of kinase activation mechanisms.

PP2A-B56 opposes Mps1 phosphorylation of Knl1 and thereby promotes spindle assembly checkpoint silencing

The kinetochore surveillance phosphatase PP2A-B56 dephosphorylates Knl1 to silence the spindle assembly checkpoint after all chromosomes have been correctly attached to microtubules.

The spindle assembly checkpoint (SAC) monitors correct attachment of chromosomes to microtubules, an important safeguard mechanism ensuring faithful chromosome segregation in eukaryotic cells. How the SAC signal is turned off once all the chromosomes have successfully attached to the spindle remains an unresolved question. Mps1 phosphorylation of Knl1 results in recruitment of the SAC proteins Bub1, Bub3, and BubR1 to the kinetochore and production of the wait-anaphase signal. SAC silencing is therefore expected to involve a phosphatase opposing Mps1. Here we demonstrate in vivo and in vitro that BubR1-associated PP2A-B56 is a key phosphatase for the removal of the Mps1-mediated Knl1 phosphorylations necessary for Bub1/BubR1 recruitment in mammalian cells. SAC silencing is thus promoted by a negative feedback loop involving the Mps1-dependent recruitment of a phosphatase opposing Mps1. Our findings extend the previously reported role for BubR1-associated PP2A-B56 in opposing Aurora B and suggest that BubR1-bound PP2A-B56 integrates kinetochore surveillance and silencing of the SAC.

Joined at the hip: kinetochores, microtubules, and spindle assembly checkpoint signaling

Error-free chromosome segregation relies on stable connections between kinetochores and spindle microtubules. The spindle assembly checkpoint (SAC) monitors such connections and relays their absence to the cell cycle machinery to delay cell division. The molecular network at kinetochores that is responsible for microtubule binding is integrated with the core components of the SAC signaling system. Molecular-mechanistic understanding of how the SAC is coupled to the kinetochore-microtubule interface has advanced significantly in recent years. The latest insights not only provide a striking view of the dynamics and regulation of SAC signaling events at the outer kinetochore but also create a framework for understanding how that signaling may be terminated when kinetochores and microtubules connect.

Kinetochore signalling: the KIss that MELTs Knl1

Mitotic kinetochores coordinate chromosome bi-orientation and anaphase onset by serving as scaffolds for the recruitment of regulatory proteins. Three new studies reveal that multiple interaction motifs of the kinetochore protein Knl1 cooperate to assemble signaling complexes that regulate chromosome segregation.

The dynamic protein Knl1 - a kinetochore rendezvous

Knl1 (also known as CASC5, UniProt Q8NG31) is an evolutionarily conserved scaffolding protein that is required for proper kinetochore assembly, spindle assembly checkpoint (SAC) function and chromosome congression. A number of recent reports have confirmed the prominence of Knl1 in these processes and provided molecular details and structural features that dictate Knl1 functions in higher organisms. Knl1 recruits SAC components to the kinetochore and is the substrate of certain protein kinases and phosphatases, the interplay of which ensures the exquisite regulation of the aforementioned processes. In this Commentary, we discuss the overall domain organization of Knl1 and the roles of this protein as a versatile docking platform. We present emerging roles of the protein interaction motifs present in Knl1, including the RVSF, SILK, MELT and KI motifs, and their role in the recruitment and regulation of the SAC proteins Bub1, BubR1, Bub3 and Aurora B. Finally, we explore how the regions of low structural complexity that characterize Knl1 are implicated in the cooperative interactions that mediate binding partner recognition and scaffolding activity by Knl1.

A Bub1-Mad1 interaction targets the Mad1-Mad2 complex to unattached kinetochores to initiate the spindle checkpoint

A Bub1–Mad1 interaction targets the Mad1–Mad2 complex to unattached kinetochores to initiate the spindle checkpoint.

Recruitment of Mad1–Mad2 complexes to unattached kinetochores is a central event in spindle checkpoint signaling. Despite its importance, the mechanism that recruits Mad1–Mad2 to kinetochores is unclear. In this paper, we show that MAD-1 interacts with BUB-1 in Caenorhabditis elegans. Mutagenesis identified specific residues in a segment of the MAD-1 coiled coil that mediate the BUB-1 interaction. In addition to unattached kinetochores, MAD-1 localized between separating meiotic chromosomes and to the nuclear periphery. Mutations in the MAD-1 coiled coil that selectively disrupt interaction with BUB-1 eliminated MAD-1 localization to unattached kinetochores and between meiotic chromosomes, both of which require BUB-1, and abrogated checkpoint signaling. The identified MAD-1 coiled-coil segment interacted with a C-terminal region of BUB-1 that contains its kinase domain, and mutations in this region prevented MAD-1 kinetochore targeting independently of kinase activity. These results delineate an interaction between BUB-1 and MAD-1 that targets MAD-1–MAD-2 complexes to kinetochores and is essential for spindle checkpoint signaling.